Yankee Stadium Mass Details and "The Concert of Hope" - Pope2008 has the precise schedule and rules regarding admission:
Visitors will not be allowed to bring metal, plastic, or glass containers into the stadium, nor backpacks, strollers, totes, large bags, food, beverages, video cameras, or tripods. Diaper bags will be permitted. Still cameras are permitted.(In other words, want to eat between 9:00am-6:00pm? Bring cash).